(1) Field of the Invention
This invention relates to a novel pressure sensitive recording unit making use of a coloration reaction through oxidation. More specifically, it relates to a novel pressure sensitive recording unit which, when applied as pressure sensitive recording paper, can provide pressure sensitive recording paper extremely superior in quality to pressure sensitive recording paper which relies on an acid-base coloration reaction.
(2) Description of the Prior Art
The pressure sensitive recording system has already been completed on the basis of electron donative colorless chromogenic compounds and acidic color-developing materials while using such colorless chromogenic compounds in the form of microscopic capsules enclosing oil droplets which in turn contain one or more of such colorless chromogenic compounds dissolved therein. It is now sold as pressure sensitive recording paper. It has established a wide-spread commercial utility for chits, slips and vouchers because it does not smudge hands and clothing, contrary to conventionally employed carbon paper.
Accompanied with the improved efficiency and manpower cutting in office work and popularization of computers, the adoption of such pressure sensitive recording paper has been promoted for a wide variety of applications. Recent increase in its sales is remarkable.
Pressure sensitive recording sheet is prepared by disposing a sheet coated with microscopic capsules enclosing fine droplets of a hydrophobic, non-volatile solvent which contains an electron donative colorless chromogenic compounds (hereinafter referred to simply as "pressure sensitive dyestuff") dissolved therein (said sheet is generally called "back-coated topsheet" and will hereinafter be abbreviated to "CB-sheet") and another sheet coated with a coating composition containing an acidic color-developing agent (called generally "front-coated undersheet" and will hereinafter be shortened to "CF-sheet") with their coated surfaces confronting each other so that the microscopic capsules can be ruptured by writing, marking or typing pressures exerted thereon through hand-writing or by means of a typewriter or any of various mechanical printers to release the pressure sensitive dyestuff-containing solution, which upon contact with the acidic color-developing agent undergoes a chemical reaction, whereby producing a color and thus a recorded image. Therefore, it is possible to make a number of copies by alternatingly superposing layers of microscopic capsules and layers of a color-developing agent.
In addition to the above-described pressure sensitive recording sheet, other pressure sensitive recording sheets of varied structures have been put to practical use and suitably selected depending on their application fields. Among such pressure sensitive recording sheets, may be mentioned a single-sheet type pressure sensitive recording sheet (hereinafter abbreviated to "SC-sheet") which is obtained by applying onto a same surface of a sheet both microscopic capsule layer and color-developing layer in two layers or coating on a surface of a sheet a mixture of such microscopic capsules and a color-developing agent so that the resulting sheet is provided with an ability to produce a color upon application of a pressure thereto in the form of either a single sheet or a plurality of superposed sheets; and a self-contained CB-sheet obtained by further coating such microscopic capsules on the back surface of the SC-sheet.
In pressure sensitive recording paper making use of such acid-base coloration systems, 3,3-bis-(4'-dimethylaminophenyl)-6-dimethylaminophthalide (common name: Crystal Violet Lactone, hereinafter abbreviated to "CVL") was initially employed as a pressure sensitive dyestuff by dissolving same in a hydrophobic solvent such as polychlorinated biphenyl or the like and then microencapsulating the thus-prepared CVL-containing solution. On the other hand, early-stage color-developing agents were led by clay minerals such as attapulgite. Since then, various improvements have been effected to both pressure sensitive dyestuffs and color-developing agents. As a result, the following pressure sensitive dyestuffs and acidic color-developing agents have been put to practical use: (1) pressure sensitive dyestuffs--a wide variety of fluorene dyestuffs, indolylphthalides and Rhodamine lactams, etc.; and (2) acidic color-developing agents--phenols and biphenols both of which may be substituted, oil-soluble acidic phenolic polymers, metal-modified phenolic polymers, derivatives of organic carboxylic acids, etc.
Although these extensively used pressure sensitive recording papers, which make use of an acid-base coloration system, are satisfactory in promptly providing deep images of various hues on CF-sheets by pressures such as writing pressures, they are on the other hand accompanied by serious common drawbacks that (1) the color fastness of developed images is not sufficient and developed images are extremely susceptible to vanishing, fading and/or discoloration during their storage over a long period of time, upon exposure to light, upon contact with an oxidizing compound (either liquid or gaseous) or solvent, especially, a polar solvent, and/or in the course of the ir storage at high temperatures; (2) the color-developing agents applied on CF-papers tend to give inconvenient yellow tinge or color to the CF-papers during their storage due to, presumably, an oxidation reaction and many of the color-developing agents tend to be deteriorated in their color-developing ability due to, also presumably, oxidation or adsorption of gases in the air; and (3) such pressure sensitive papers are costly since they use costly dyes and color-developing agents are coated in a great quantity on base web sheets such as papers. Accordingly, there is a standing desire for the development of a color-producing system overwhelmingly superior to those employed in the conventional pressure sensitive recording sheets and a pressure sensitive recording material making use of such a color-producing system.
A wide variety of substitutes for conventional pressure sensitive recording systems utilizing an acid-base coloration have been studied, including the following proposals:
(1) Use of a color-developing reaction through chelation between a metal salt and ligand (Japanese Patent Publication Nos. 28730/1969 and 5616/1970);
(2) Method making use of the color-producing phenomenon owing to the oxidation of diphenylmethane dyestuffs (Japanese Patent Publication No. 5625/1963);
(3) Pressure sensitive recording paper employing bis-(2,4-dinitrophenyl) acetate as a dyestuff precursor and an alkaline color-developing agent (U.S. Pat. No. 4,113,282);
(4) Attempted application of the color-producing reaction through the formation of a charge-transfer complex in pressure sensitive recording paper (Tappi 56, No. 8, 1975, PP 128-132);
(5) Application of a coupling reaction of a diazo compound in pressure sensitive recording paper (Japanese Patent Publication No. 32368/1974; Japanese Patent Laid-open No. 85811/1976).
However, none of the above proposals have been found successful for the following reasons (the item numbers in brackets correspond to the item numbers of the above proposals):
(1) It is difficult to obtain sharp images of various colors, except for black color. If the materials used for inducing the chelate reaction are water-soluble substances, water-in-oil type microscopic capsules are required and problems are thus encountered regarding the preparation of such microscopic capsules, their application onto base web sheets and the quality of resulting pressure sensitive recording paper. Moreover, ligands used in chelate reactions are generally liable to decomposition coloration through their oxidation or reduction by heat, light, moisture, etc. Therefore, pressure sensitive recording paper according to proposal (1) has not been used extensively;
(2) This proposal is not suitable for practical use as the diphenyl methane dyestuffs have considerable sublimability, the density of produced colors is not sufficient and produced color images are unstable (namely, tend to discolor or fade) during their storage;
(3) Pressure sensitive recording paper according to proposal (3) suffers from an extremely slow color-producing speed, whereby making itself unsuitable for practical use;
(4) The color-producing reaction takes place through the formation of a complex by a donor (electron donor) of a charge-transfer complex and its acceptor (electron acceptor). Therefore, resulting pressure sensitive recording paper involves such problems that the density of a produced color is low and developed color images have extremely low stability to light or heat. Such pressure sensitive recording paper is accordingly not suitable for practical use; and
(5) Pressure sensitive recording paper according to proposal (5) is difficult to produce a blue.about.black hue which is generally preferred. It also involves problems with respect to color-developing speed storage and resistance to light. Thus, it has not been put to practical use.
In pressure sensitive recording paper comprising an acid-base coloration system, namely, CVL and an acidic color-developing agent, particularly, an acid clay type color-developing agent such as acid clay per se, benzoylleucomethylene blue (BLMB) is additionally incorporated to suppress the phenomenon that developed color images are faded too early. However, the incorporation of BLMB is accompanied by another drawback that, since this dyestuff produces a greenish blue color, the hue of a color image developed on a CF-sheet coated with acid clay tends to become greenish along the passage of time as CVL is faded. As a countermeasure for the above drawback, it was proposed to use a methine-type dyestuff represented by a triphenylmethane dyestuff in conjunction with CVL (see, Japanese Patent Publication Nos. 5134/1959 and 11991/1966). However, many of these methine-type dyestuffs are extremely unstable during their storage. Therefore, a microscopic capsule suspension prepared for their application in pressure sensitive recording paper is considerably colored during its microencapsulation step. In addition, upon exposure to light, pressure sensitive recording paper coated with such microscopic capsules tends to readily develop a color by itself. For these reasons, the incorporation of such methine-type dyestuffs has not been carried out in an industrial scale. Moreover, such triphenylmethane dyestuffs have been considered to be absolutely unsuitable as color reactants for practical application because their color-producing speeds upon contact with an acidic color-developing agent (acid clay, attapulgite, phenol-formaldehyde polymer, or the like) is extremely slow (see, for example, Hiroyuki Moriga, "Introduction to Chemistry of Special Paper", Kobunshi-Kankokai, Kyoto, Japan, 1975, P 46).
On the other hand, methine-type dyestuffs having at one or more portion thereof one or more heterocyclic rings of a large molecular weight are relatively stable during storage thereof. It has also been proposed to employ such a methine-type dyestuff as an auxiliary color reactant used together with CVL for pressure sensitive recording paper, in combination with an acid clay-type color-developing agent. However, methine-type dyestuffs of this sort suffer from extremely slow color-producing speeds thereof.
Moreover, the acid clay, a color-developing agent, per se has tendency of lowering its color-producing ability along the passage of time. These problems have not been solved at all. Thus, they have not become a drastic solution to the problems of conventional pressure sensitive recording paper which depends on an acid-base color-producing system.